Keys made short work of Vandeweghe whose Grand Slam year is now a case of what might have been after a 66-minute defeat to the 15th seed whose only previous major semi-final was the 2015 Australian Open where she was beaten by Serena Williams.

"Oh, my God, it feels so good". On top of coming back after having a big win and then all of a sudden being in a match where you're supposed to win, it was a lot - and then being the last American.

"I've known her for a long time".

The last all-American final in a major was in January, when a pregnant Serena Williams defeated her sister, Venus, at the Australian Open.

"I practised with her quite a bit and sometimes I would get frustrated because things came so easily to her that it looked lazy".

It's the first time four Americans have made the women's semifinals at the US Open since 1981, when Tracy Austin beat Martina Navratilova for the title.

The last time it happened at any Grand Slam tournament was at Wimbledon in 1985, with Navratilova and Evert joined by Zina Garrison and Kathy Rinaldi.

While Williams is 37, the oldest semifinalist in tournament history, the other members of the remaining trio are all in their 20s. She will move to 22nd if she loses, and possibly as high as 15th if she defeats compatriot Madison Keys to win the title.

Keys set the tone in the opening game: ace at 108 miles per hour, ace at 105 miles per hour, service victor at 117 miles per hour, service victor at 112 miles per hour.

"I was real nervous", Keys said.

"I can't tell you how many times I have sat in this chair and had to hear how disgusting tennis is in America".

The 25-year-old Vandeweghe equals her best showing a major.

Venus Williams of the US reacts winning a point against Czech Republic's Petra Kvitova during their 2017 US Open Women's Singles Quarterfinal match at the USTA Billie Jean King National Tennis Center in NY on September 5, 2017.

After wrapping up victory, Vandeweghe briefly fell to her knees before walking over to her box to shake hands and exchange words with her coach, former Wimbledon champion Pat Cash.

"Let's make it four from four", said Vandeweghe.

"You will have to ask him how he's been able to do that".

Wednesday's victory was Vandeweghe's second over a World No.1 in her career.

Vandeweghe wound up with more aces, 5-2, and even produced three service winners. "Whether it was not a great return that just got over the net, I know as a big server it's really annoying when your serve keeps coming back". "I'm just happy to be in the final". After beating Vandeweghe in the Stanford final, Keys jumped into her opponent's arms and then sat on her lap.

"Feeling like I was getting on a roll definitely helped relax and loosen up to play some of my best tennis", Keys said. I couldn't feel better than I feel right now.

Madison Keys, who has been fighting an original wrist surgery in November, only re-joined the tour in March.

The 25-year-old Vandeweghe will play the victor of Wednesday night's semifinal between Estonian qualifier Kaia Kanepi and No. 15 seed Madison Keys of the United States.

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